Simulate Failsafes

Failsafes define the safe limits/conditions under which you can safely use PX4, and the action that will be performed if a failsafe is triggered (for example, landing, holding position, or returning to a specified point).

In SITL some failsafes are disabled by default to enable easier simulation usage. This topic explains how you can test safety-critical behavior in SITL simulation before attempting it in the real world.

::: info You can also test failsafes using HITL simulation. HITL uses the normal configuration parameters of your flight controller. :::

Data Link Loss

The Data Link Loss failsafe (unavailability of external data via MAVLink) is enabled by default. This makes the simulation only usable with a connected GCS, SDK, or other MAVLink application.

Set the parameter NAV_DLL_ACT to the desired failsafe action to change the behavior. For example, set to 0 to disable it.

::: info All parameters in SITL including this one get reset when you do make clean. :::

RC Link Loss

The RC Link Loss failsafe (unavailability of data from a remote control) is enabled by default. This makes the simulation only usable with either an active MAVLink or remote control connection.

Set the parameter NAV_RCL_ACT to the desired failsafe action to change the behavior. For example, set to 0 to disable it.

::: info All parameters in SITL including this one get reset when you do make clean. :::

Low Battery

The simulated battery is implemented to never run out of energy, and by default only depletes to 50% of its capacity and hence reported voltage. This enables testing of battery indication in GCS UIs without triggering low battery reactions that might interrupt other testing.

To change this minimal battery percentage value use the parameter SIM_BAT_MIN_PCT.

To control how fast the battery depletes to the minimal value use the parameter SIM_BAT_DRAIN.

:::tip By changing SIM_BAT_MIN_PCT in flight, you can also test regaining capacity to simulate inaccurate battery state estimation or in-air charging technology. :::

It is also possible to disable the simulated battery using SIM_BAT_ENABLE in order to, for example, provide an external battery simulation via MAVLink.

Sensor/System Failure

Failure injection can be used to simulate different types of failures in many sensors and systems. For example, this can be used to simulate absent or intermittent GPS, RC signal that has stopped or got stuck on a particular value, failure of the avoidance system, and much more.

For example, to simulate GPS failure:

Enable the parameter SYS_FAILURE_EN.
Enter the following commands on the SITL instance pxh shell:

```sh # Turn (all) GPS off failure gps off

# Turn (all) GPS on failure gps ok ```

See System Failure Injection for a list of supported target sensors and failure modes.